In a significant step for the largest reservoir project in the Bay Area in 20 years, workers have finished building the spillway — a massive concrete channel as wide as eight lanes of freeway and a quarter mile long — at Calaveras Dam near the Alameda-Santa Clara county line.

The $810 million project to replace the old dam with a new, more earthquake-proof version has been beset by delays and cost overruns, due to the discovery of ancient landslides and other difficulties in the years since work began in 2011 that have made the project more complicated.